1. Field of the Invention
The present invention relates to a head up display device for a vehicle.
2. Description of Related Art
Conventionally, a head up display device (hereinafter referred to as an HUD device) for a vehicle is known. The HUD device displays a light-emitting image on a display, and projects the display image onto a projection member such as a windshield. Accordingly, the HUD device displays a virtual image of vehicle-related information. As a type of such a HUD device, a device that reflects a display image of a display on a reflecting mirror such as a concave mirror and projects the reflected image onto a projection member is disclosed in JP-A-2009-132221. As described above, an installation space occupied by the HUD device in a vehicle is made small by the use of the reflecting mirror.
In an HUD device disclosed in JP-A-2009-132221, the following configuration is employed for adjusting a display position of a virtual image. That is, a drive signal in accordance with an adjustment command from the outside is applied to a stepping motor, and a rotatable reflecting mirror is rotated by the stepping motor. As a result of such a configuration, by giving the adjustment command to the HUD device, an occupant of a vehicle can display the virtual image of vehicle-related information in an easily visually identifiable state.
In the above-described HUD device disclosed in JP-A-2009-132221, the display position may be smoothly adjusted in accordance with the adjustment command to relate the display position of the virtual image finely to preference of a vehicle occupant. For a method for smoothly adjusting the display position of the virtual image in accordance with the adjustment command, a method for micro-step controlling the drive signal of the stepping motor, such that an electrical angle changes each time by a set angle that is smaller than an interval between stabilizing points, which appear at every predetermined electrical angle in accordance with detent torque in a stepping motor, is demanded. This is because significant cost rising can be reduced as compared to mechanical processes, such as the increase in the number of steps of the stepping motor or increase of a reduction ratio (gear ratio) on a motor output side through a reduction gear mechanism, since the micro-step control can be dealt with through electrical processing.
However; in the case of the above-described micro-step control, if the present electrical angle is out of the stabilizing point at the time of the stop of the adjustment command, a rotation retaining effect due to the detent torque is decreased, so that the stepping motor may lose steps because of impact force by such as external force action or vibration. In the stepping motor that has lost steps, discrepancy between a mechanical angle corresponding to a rotation angle of the reflecting mirror and the electrical angle is caused. Accordingly, reliability for the adjustment of the display position of the virtual image is decreased.